The present invention is directed to a flexible shaft coupling having a cylindrical elastomeric member within internal slots centrally located between a pair of shaft hubs each having teeth axially extending toward the other hub.
Flexible couplings are used to transmit power or torque between a driving element, normally a shaft connected to a prime mover, and a driven element, normally a shaft connected to a load. The flexible coupling adjusts for both parallel and angular misalignment of the two shafts. One common form of flexible coupling has a central elastomeric element transmitting the torque between a driving hub and a driven hub. The term xe2x80x9chubxe2x80x9d as used herein includes an axially located member directly connected to the driving or driven device, or integral with the shafts of the driving or driven device, or most commonly mounted on the shafts of the driving or driven device by means of keyways so as to transmit torque. Each hub has axially extending teeth which do not directly engage teeth of the other hub but are received in radial slots of a centrally located cylindrical elastomeric element so that the torque is transmitted through the elastomeric element and misalignment can be tolerated. The elastomeric element also, due to its partial compressibility, softens the transmission of torque from one hub to the other upon acceleration or deceleration.
The preferred form of elastomeric element has a radial slit which permits the elastomeric element to be unwrapped from its normal cylindrical shape. The slit allows the elastomeric element to be unwrapped or opened, positioned adjacent to the teeth of the hubs, and then wrapped or closed around the teeth so as to be in direct physical contact and engage the teeth of both hubs to transmit torque. This permits removal of the elastomeric element without requiring movement of the hubs, and more importantly movement of either the prime mover or the load connected by the flexible coupling. In its most common form the elastomeric element is surrounded by an outer sleeve which prevents the cylindrical elastomeric element, when the coupling is in an assembled state, from either unwrapping or expanding outwardly when under load or rotation.
It is important that the elastomeric element transmitting the torque between the hubs, and being in shear under a loaded condition, be designed in a manner which provides sufficient flexibility to permit the unwrapping and wrapping of the elastomeric element and also reduce any tearing of the elastomeric element by the shear force asserted on the elastomeric element when it is transmitting power from the driven hub to the driving hub. It is also important that the slots of the elastomeric element and the hub teeth be designed in a manner to permit full engagement with area contact between the elastomeric element slots and the hub teeth when the elastomeric element is located on the teeth in the assembled condition.
It is an object of the present invention to increase the life of the elastomeric element of a flexible coupling by having the slot bottom wall substantially tangential to the outer periphery of the elastomeric element. Such provides relatively uniform thickness of the elastomeric element between the bottom wall of the slot and the elastomeric member outer surface. Furthermore, the bottom wall of the slot is joined to the slot side walls by compound curved walls so that there are no sharp corners between the slot bottom wall and the slot side walls with the compound curved walls being located so that the bottom portion of the elastomeric member slots is wider or thicker than the remainder of the slots. Preferably, the radius of the concave wall portion joining the slot bottom wall to the slot side walls is 15%-25% of the width of the slots, 3 to 7 times clearance between the outer periphery of the hub teeth and the bottom wall of the elastomeric element, and 5% to 15% of depth of the elastomeric element slot minus the aforesaid clearance. This form of construction creates greater flexibility of the elastomeric element near the bottom of the slots while reducing stress concentration at the location where the slot side walls joint the slot bottom wall. This furthermore increases the flexibility in the elastomeric element which allows the slot side walls to maintain better area contact with the hub teeth surfaces.
Thus it is an object of the present invention that the elastomeric element slots have a pair of slot side walls and a slot bottom wall and each of the slot side walls being joined to the slot bottom wall by a concave wall with one end of the concave wall being tangential to the slot bottom wall and the other end of the concave wall blending in with the slot side walls. Even more preferably, the concave wall joining the slot bottom wall and side walls is joined to the slot side walls by a second convex wall. In the preferred form of the present invention, the hub teeth outer periphery does not engage the slot bottom wall but is located in the relief area formed by the concave walls near the bottom of the elastomeric element slots. Thus, the sharp peripheral outer edges of the teeth do not engage the elastomeric element even when torque is applied to the flexible coupling, and this also reduces stress on the elastomeric element to further increase its life.
The present invention is also directed to a flexible coupling of the type described wherein the side surface of the hub teeth are tapered or curved toward the free end of the teeth. This taper of large radius creates a smooth outer surface near the free end of each tooth where the teeth first contact the elastomeric element. This reduces the stress on the elastomeric element at the free end of the hub teeth by allowing area contact instead of linear contact when the coupling is placed under load. This results in a reorientation of the pressure distribution from the hub teeth to the elastomeric element slots resulting in more central location for reduced bending stress. This improves the flexible coupling""s ability to accommodate both angular and parallel misalignment. Therefore, it is an object of the invention that the hub teeth and the flexible element of the type described have tapered or curved surfaces adjacent to the free end of the teeth.
Although flexible couplings of the type described are generally assembled with both a driving shaft and driven shaft already in place and the central elastomeric element wrapped around the hub teeth, it is sometimes desirable that the flexible coupling is assembled by axially moving one hub toward the other hub with the hub teeth entering and engaging the slots of the elastomeric element without requiring that the elastomeric element be unwrapped. Therefore, it is an object of the present invention that the hub teeth of the coupling have tapered or curved surfaces near the free end of the hub teeth permitting the flexible coupling to be assembled by locating the first and second hubs on the coupling axis with the elastomeric element being located on the teeth of one of the hubs and axially inserting the teeth of the second hub by relative axial movement of the second hub relative to the first hub.